The present invention relates to silicon with a high oxygen content and a high dislocation density, to its production and to its use for the production of solar cells.
Crystalline silicon is the material from which the vast majority of all solar cells for photovoltaic conversion of sunlight into electrical energy are currently manufactured. Monocrystalline and polycrystalline silicon form the two principle variants of the silicon material used for solar cells. While monocrystalline silicon is usually pulled as a single crystal from a silicon melt using the Czochralski process, there are a number of production processes for polycrystalline silicon. The most usual processes are various block-crystallization processes in which the silicon wafers for producing the solar cells are obtained by sawing a solid polycrystalline silicon block, and various film-drawing processes or film-casting processes, in which the wafers are drawn or cast in their final thickness as a silicon film from a molten material. Examples of the film-drawing process are the EFG process (Edge-defined Film-fed Growth) (EP 0,369,574 A2) and the RGS (Ribbon Growth on Substrate) process (EP 0,165,449 A1, DE 4,102,484 A1, DE 4,105,910 A1).
Solar cells are large-area pn diodes, in the volume of which the sunlight generates minority charge carriers which have to diffuse towards the emitter at the surface of the cell, so that they can there be separated at the pn-junction by the electric field and contribute to the external current flow. The greater the service life and therefore also the diffusion length of the minority charge carriers in the base, the more effective this process. Consequently, particular demands are imposed on the quality of the silicon material for producing solar cells: this material must be as far as possible free of impurities and crystal defects, in order to enable a maximum diffusion length of the minority charge carriers to be achieved.
Oxygen is usually the dominant impurity in silicon, since the silicon melt is usually melted in a quartz crucible (SiO.sub.2). Although oxygen is electrically inactive as an interstitially dissolved impurity, it is known that if there is an oxygen content of above approximately 8.times.10.sup.17 atoms/cm.sup.3 in the silicon material caused by high-temperature steps, the minority carrier service life is also reduced (J. Vanhellemont et al., J. Appl. Phys. 77 (11), 5669 (1995)), and therefore so is the efficiency of solar cells. Therefore, the rule has hitherto been that silicon material with an oxygen content of over 8.times.10.sup.17 atoms/cm.sup.3 is unsuitable for the production of solar cells.
Especially for highly productive film-drawing or film-casting processes, it is particularly difficult and expensive to produce a silicon material with a sufficiently low oxygen content. Particularly in the case of the RGS material, it is necessary to apply a stream of oxidizing gas to the solidifying surface in order to establish a sufficiently smooth surface (DE 4,105,910 A1). Consequently, a relative high oxygen concentration is established in the volume of the RGS material. In the layer of silicon which is close to the surface, the oxygen content is even greater, and in fact the surface of untreated specimens is even to a large extent covered with a layer of silicon dioxide. This is necessary and desirable in order to immobilize impurities which have segregated out. In the case of thin silicon films such as an RGS film having a thickness of around 300 .mu.m, without the getting action of the oxygen close to the surface, the metallic impurities which have just been concentrated in this thin zone would immediately diffuse back into the interior of the film, which is still at a temperature of over 1000.degree. C.
In view of the foregoing, the invention is based on the object of preparing silicon which, despite having a high oxygen concentration, is suitable for use in photovoltaics. Advantageously, this allows solar cells which contain this silicon to reach levels of efficiency which enable them to be used economically.
Surprisingly, it has now been discovered that silicon with a high oxygen concentration can be used in photovoltaics if it additionally has a high density of crystal lattice dislocations (referred to below as dislocations). This is even more surprising since a high dislocation density generally leads to materials which are relatively unsuitable for photovoltaics.